Hydraulic cylinder piston and rod combinations are being increasingly used in many fields, a principal one of which involves the moving and positioning of material and objects. As the various applications of hydraulic cylinder piston and rod combinations have progressed, more stringent operational criteria are being encountered and systems are appearing in the art that precisely, reliably, and continuously sense the position of the piston and its related displacement parameters, velocity and acceleration, through correlation of resonances of electromagnetic waves in the cavity that is made up of the cylinder and the piston, with the dimensions of the cavity. In some such systems the wave performance is similar to that of an electrical transmission line with shorted ends, in that the resonance frequency of an electromagnetic standing wave correlates inversely with the cavity length.
In the hydraulic cylinder cavity, there is a high loss of a transmitted signal except at a resonance frequency corresponding to a piston position or cavity length where energy is coupled into a particular mode standing wave. At resonance, the amplitude rises sharply with frequency to a peak then declines. The frequency at resonance is lower when the cavity is longer and higher when the cavity is shorter. Correspondingly, the amplitude of each resonance will vary with the cavity length; the lower amplitudes being at the lower resonant frequency and the higher amplitudes being at the higher resonant frequency.
There are variations in position sensing systems in the art such as: injecting the electromagnetic signal into the cavity on the rod or on the head side of the piston; the excitation of particular mode standing waves; and the sensing of a harmonic other than the fundamental of a particular resonance frequency. As illustrations of progress in the art, in U.S. Pat. No. 4,737,705 a coaxial resonant cavity is employed within which the electromagnetic waves are launched and propagated in the mode referred to as the transverse electromagnetic wave (TEM) mode. The cylinder on the rod side of the piston is one type of coaxial cavity.
In U.S. Pat. No. 5,182,979, compensation through an equalization capability is provided for differences in losses as the extension of the piston and rod in the cylinder takes place.
In U.S. Pat. No. 5,325,063, identification of a fundamental resonance frequency is enhanced by the use of positive and negative slope intercepts of a resonance signal with respect to a reference level, where the positive slope is at the lower frequency and the negative slope is at the higher frequency.
In U.S. Pat. No. 5,617,034, an improved output signal is achieved by placing the input and output electromagnetic wave couplers at approximately 90 degree angular separation on the periphery of the hydraulic cylinder. Unwanted mode resonance is suppressed by positioning dummy couplers at further 90 degree positions.
In U.S. Pat. No. 5,608,332, increased sensitivity is achieved through the use of automatic gain refinement capability.
In U.S. Pat. No. 5,710,514, increased piston position accuracy is achieved by compensating for piston velocity in the determination of the piston position.
In copending application Ser. No. 08/799,280, filed concurrently herewith (Caterpillar File 94-453), increased piston position accuracy is achieved by compensating for changes in the dielectric properties of the hydraulic fluid in the piston position system.
The hydraulic cylinder piston and rod combinations may frequently be used in isolated and severe environmental conditions.